1. Field of the Invention
This invention pertains to fixed offshore platform construction and installation and, more specifically, to a simplified method of installing fixed offshore platforms using a preinstalled piled foundation, thereby avoiding the use of jacket legs and skirt pile sleeves as templates for guiding the placement of the pilings.
2. Description of the Prior Art
Offshore platforms can be roughly categorized as being submersible, semisubmersible, jackup and fixed. Although they all have their advantages and advocates, the fixed platform construction is in widespread use. That is true even though in many deeper water installations, the materials and the installation procedures are both quite expensive.
Although there are many variations, the conventional installation procedure begins with the placement of the jacket. The jacket is a tubular framework which includes a plurality of sleeves, some of which are used for guiding the pilings during installation. In addition, the jacket includes a mudmat portion which is the horizontal support framing that forms a stable base for temporarily supporting the jacket vis-a-vis the mudline or seabed during installation. The jacket structure has a vertical dimension sufficient to allow its bottom to rest on the sea bed where the mudmats are located so that its top section will be several feet, usually 10-15 feet, above the waterline. If the distance between seabed and waterline is several hundred feet, then the jacket is several hundred feet tall.
The legs of the jacket are tubular sleeves. The main pilings are driven through these sleeves, which are located at the corners and sometimes between the corners of the jacket so as to establish a firm foundation anchored into the subterrain or geological formation under the mudline. It will be seen that if the sleeves are hundreds of feet long, then the main pilings will have to be a great deal longer to permit the pilings to pass through the sleeves and then extend a great distance into the subterrain. The jacket may also include additional sleeves that do not come all of the way to the waterline for accepting skirt pilings. Such pilings are sometimes necessary to add additional support for the jacket and in many cases the skirt pile sleeves are 100 feet long or more above the mudline.
The main pilings and the skirt pilings are then grouted through the annulus of the jacket sleeves. The grouting is conventionally a cement slurry without aggregate that is fed through appropriate tubular conduits to seal the jacket sleeves to the pilings. Finally, the deck is installed to the top of the jacket, typically by welding. It may be seen that both the compression load and the tension load on the platform resulting from wave and wind action and structural and equipment weight will be carried by the grouting.
As mentioned previously, the jacket structure can be enormous in size for installations made in deeper water. Such a jacket is normally fabricated shoreside and includes at least temporary bulkheading of the sleeves so that the structure can be made buoyant. Hydraulic tubing and valving is also provided. Typically, the larger sized jacket is towed on its side on a barge to the installation site, then launched from the barge, and via the valving, the structure is selectively flooded so that the jacket turns and descends to its installed position. The pilings are then driven into place. When the pilings are driven through the jacket legs or skirt pile sleeves, the bulkheading included in the legs or sleeves is penetrated and destroyed.
It should be noted that the dimensional requirements for the framework of the jacket is determined for deep-water installations primarily by the foundation anchoring requirements caused by the soil and the geological conditions at the site, not by the water depths or the current conditions. That is, the foundation stability requirements usually require larger piling sleeve diameters than would be required merely to support the platform and the platform load in the presence of the water conditions.
To further demonstrate the approximate magnitude of exemplary prior art installations, at least one installation, the Exxon Hondo installation, utilized a jacket that was so long, approximately 850 feet, that it was not practical to fabricate the jacket totally in one piece at that time at a shoreside location. The jacket was made in two parts, each over 400 feet long, and the two pieces were separately towed to the installation location. Here, the legs or the sleeves were mated and welded while the jacket parts were in their floating positions. Then, the jacket was selectively flooded in the manner described above so that it descended to its installed position.
The Shell Cognac jacket structure was made in three sections since this installation required a jacket length of over 1,000 feet. The lower segment was installed so as to form a permanent template for the pilings. The pilings were then driven through the jacket legs of this structure and grouted. The grouting alone filled the annulus around the pilings up through the sleeve in the lower segment. Then the second segment was positioned and mechanically connected to the first and then the top section or segment was positioned and mechanically connected to the second segment. Pin piles were inserted and grouted to connect the second segment to the first segment and to connect the third segment to the second segment. Then the deck was installed on the top of the jacket in conventional fashion. Again, the grouting carried the entire compression and tension loads.
Main pilings extending above the water line are not typically employed in deeper water installation, such as the Shell Cognac installation. The skirt pilings that are employed in the lower jacket segment of these so-called "tower" platforms do extend above the mudline at least about 100 feet or more, however.
Another prior art tower platform that is known is the Shell Bullwinkle platform. This structure is being installed in 1,400 feet of water using a conventional one-piece jacket, but no main pilings. Instead, the jacket includes sleeves for accepting 28 skirt pilings of significant length.
Another procedure to that described above is described in U.S. Pat. No. 3,528,254, J. R. Graham, issued Sept. 15, 1970. This patent shows a structure that is basically a mobile system which is at least semi-permanently installed. In this structure, a permanently installed jacket is employed which does not reach all of the way to the waterline, but to a fixed distance below the waterline. The vertical column members attached to the decking are positioned with respect to the top of the jacket and secured by a locking arrangement, best shown in FIG. 7. It should be noted that the piling below the locking arrangement is grouted in conventional fashion. When movable platform structure 15 is to be removed, the locking arrangement permits this. The jacket is then abandoned since it is permanently installed and the movable structure is towed to a new location where a similar jacket has already been pre-installed. It should be noted that the top of the jacket at the new location must be approximately the same distance below the waterline as the top of the jacket at the first location in order for platform deck height to be properly vertically positioned.
It is believed that were this feature ever utilized, the jacket also would have to be removed or destroyed since it could not be left in place to be a hidden hazard to shipping.
Therefore, it is a feature of the present invention to provide an improved method of constructing an fixed offshore platform using a preinstalled piled foundation.
It is another feature of the present invention to provide an improved method of installing a fixed offshore platform, which avoids using a jacket as a template for locating and guiding the pilings.
It is still another feature of the present invention to provide an improved method of installing a fixed offshore platform which eliminates many of the components of conventional fixed offshore platform foundations, such as mudmats and skirt pile sleeves.
It is still another feature of the present invention to provide an improved fixed offshore platform which eliminates the need for oversizing the support structure to conform to piling size necessary for soil conditions, making it only necessary to size the support structure to conform to the stresses applied to the structure because of water and wind environmental conditions and because of the mass of the deck and equipment carried by the platform.
It is still another feature of the present invention to provide an improved fixed platform structure which can be reused and which is not partly bulkhead damaged, so that its vertical columns can readily be rendered buoyant for removal and towing purposes.
It is still another feature of the present invention to provide an improved fixed platform structure that can be comparatively easily removed so that the remaining structure is not potentially hazardous to shipping and the like.